1. Field of the Invention
This invention relates to an optical amplifier to be used for an optical transmission system and, more particularly, it relates to a remotely pumped Er-doped optical fiber amplifier.
2. Prior Art
Remotely pumped optical amplifiers have been proposed for optical amplifiers to be used in submarine optical transmission systems because the pumping source to be used for the system does not have to be laid on the seabed.
While 980 nm and 1,480 nm bands are widely used for pumping light for Er-doped optical fiber amplifiers, a pumping source with the latter band, or the 1,480 nm band, is typically used for a remotely pumped amplifier system because of its low loss rate in the optical transmission path.
A remotely pumped optical amplifier has an advantage that the pumping source does not have to be laid on the seabed and a disadvantage that the intensity of pumping light is reduced before it gets to the Er-doped optical fiber due to the loss of the pumping light as it passes through the optical transmission path.
Thus, the distance between the Er-doped optical fiber and the pumping source has to be made as short as possible in order to ensure an intensity of the pumping light getting to the Er-doped optical fiber. In other words, the transmission path is subjected to a relatively short limit length.
This disadvantage on the part of the remotely pumped amplifier system can be partly overcome by increasing intensity of pumping light by use of polarization combiner and wavelength division multiplexer/demultiplexer and thereby increasing the limit distance between the pumping source and the Er-doped optical fiber.
Currently, however, only up to four semiconductor lasers can be polarization- and wavelength-multiplexed and hence there is a strong demand for means that can secure an enhanced intensity of pumping light.
In view of the above circumstances, it is therefore the object of the present invention to provide an Er-doped optical fiber amplifier adapted to increase the length of transmission path by raising the intensity of pumped light.